The retinula cells in the retina of Drosophila melanogaster have been shown to effect the development of those cells in the optic lobes which are postsynaptic to them. We propose to compare the cell morphologies of this postsynaptic population in the wild type to those in various mutants of the visual system in which this population is abnormal. Observation will be made on these cells at various times during their development as well as on the adult forms. In addition, we plan to generate genetic mosaics to observe the effects of interactions between genotypically different tissues of the eye and brain which are juxtaposed from early developmental times. Our objective is to assess the inductive effects of retinula cells on their first postsynaptic populatton by studying the effects of mutations at genetic loci which are known to control the transmission of information from the eye to the brain and by evaluating the development of cells of the optic lobe in the absence of innervating retinula fibers. We hope that this information may aid in the understanding of some of the mechanisms which control the final morphology of cells in the nervous system.